1. Field of the Invention
The present invention relates generally to nonaqueous suspensions for electrophoretic deposition of powders. More particularly, this invention relates to nonaqueous suspension for electrophoretic deposition of powders adapted for use with electrophoretic deposition of various powders such as phosphor powders for making a finely patterned color phosphor screen of a cathode ray tube, cathode material powders for the cathode of the cathode ray tube, insulating powders such as alumina or the like on a filament surface used in an indirect heating cathode of the cathode ray tube and powders for passivation of the surface of semi-conductor devices, for example, passivation films in the grooves of mesa semi-conductor devices.
2. Description of the Prior Art
For the electrophoretic deposition of powders in nonaqueous solution as described above, an electrophoretic deposition of powders in nonaqueous solution which performs a so-called anodic deposition by suspension in which nitrocellulose is dissolved into a ketonic solvent into which powders are mixed is disclosed in, for example, Japanese Examined patent application publication No. 20431/1975, Japanese Unexamined patent application publication No. 118363/1978 and so on. According to such electrophoretic deposition of powders in nonaqueous solution, superior powder deposition can be carried out as compared with a so-called cathodic deposition employing a conventional aqueous suspension for electrophoretic deposition.
Such a case in which a color phosphor screen is deposited on the glass panel of a cathode ray tube according to electrophoretic deposition will be described. In this case, a transparent electrode with a pattern corresponding to a depositing pattern of phosphor or electrode pattern made of, for example, In.sub.2 O.sub.3 or SnO.sub.2 : Sb is formed in advance on the inner surface of a glass panel and then the electrophoretic deposition of phosphor powders is carried out on this transparent electrode pattern. However, according to a conventional electrophoretic deposition of powders in aqueous solution, H.sub.2 0 contained in the suspension is decomposed to H.sup.+ and OH.sup.- in parallel with the powder deposition so that H.sup.+ is moved to the transparent electrode which is applied with the negative potential and serves as a member to be electrophoretically deposited and that H.sup.+ reacts with the transparent electrode to thereby develop or form hydrogen gas. By the gas evolution, a pin hole is caused in the phosphor powder layer thus electrophoretically deposited or the surface thereof is roughened or made coarse, that is the packing density of the phosphor layer is reduced. Further the transparent oxide electrode is reduced to degrade the electric characteristic thereof or the transparent electrode is browned. Moreover, electrolysis of water occurs in addition to the electrophoretic deposition so that current efficiency is low and it takes considerable time to obtain a phosphor layer of significant thickness. Further, metallic ions of metallic salts added to the suspension are electrophoretically deposited together with the powders, for example, Al.sub.2 O.sub.3, thus causing insufficient insulation. Also, the above metallic ions work as a killer for the electrophoretic-deposited phosphor layer to lower the brightness thereof.
On the other hand, according to the electrophoretic deposition of powders in nonaqueous solution, the suspension contains almost no water. Even if the suspension contains water, since the anodic deposition method is used, namely, the electrode to be electrophoretically deposited is supplied with positive potential, H.sup.+ generated by the electrolysis of water is moved to the opposite electrode, thus causing no undesirable phenomenon as mentioned above.
Although the electrophoretic deposition of powders in nonaqueous solution has many advantages as described above, in practice, this type of suspension is unstable (i.e. it had a short pot life) and its reproducibility was poor. These constitute the obstacles for the wide use of this kind of the electrophoretic deposition of powders in nonaqueous solution.